This article is from the Depression FAQ, by Cynthia Frazier with numerous contributions by others.
Another type of depressive disorder is manic-depressive illness, also called bipolar depression. Not nearly as prevalent as other forms of depressive disorders, manic depressive illness involves cycles of depression and elation or mania. Sometimes the mood switches are dramatic and rapid, but most often they are gradual. When in the depressed cycle, you can have any or all of the symptoms of a depressive disorder. When in the manic cycle, any or all symptoms listed under mania may be experienced. Mania often affects thinking, judgment, and social behavior in ways that cause serious problems and embarrassment. For example, unwise business or financial decisions may be made when in a manic phase.